Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Patent
1979-06-07
1982-05-18
Bell, Mark
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
501 48, 501 50, C03C 312, C03C 314
Patent
active
043306292
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to sealing materials suitable for making heat-resistant, and usually hermetic, seals between components of electrical discharge devices.
It has been known for a number of years, as revealed in U.S. Pat. No. 3,588,573 and German Pat. No. 2,307,191, that the current sealing material for high pressure sodium lamps based on a modified calcium magnesium aluminate composition with barium oxide and a small amount of boric oxide is chemically inadequate in the presence of reactive metal halide vapours at around 800.degree. C. In U.S. Pat. No. 3,588,573 a family of high temperature metal halide resistant sealing compounds using alumina and rare-earth oxides with high melting points ranging from 1720.degree. C. to 1800.degree. C. is disclosed. The use of such high temperatures for sealing the end of the arc tubes, particularly in the case of short tubes, presents considerable technical problems. For instance, it can easily cause volatilisation of the metal halide species in the final lamp processing.
German Pat. No. 2,307,191 discloses the use of some sealing compounds in the Al.sub.2 O.sub.3 -SiO.sub.2 -MnO system, which are claimed to be metal halide resistant. The silica in the sealing compound is likely to interact with certain metal halides, such as scandium iodide, commonly used in conventional metal halide lamps with a silica envelope. The reaction between scandium iodide and silica is a well-known factor limiting the colour rendition and efficiency of these lamps. The disclosure by Matshusita in Japanese Patent Application No. 47-34066 of an unspecified metal halide resistant sealing material of the Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -BeO type is also suspect because of the presence of silica. It is also likely to be objectionable in the lighting industry because of the presence of beryllia, which is a highly toxic material.
We have now found in accordance with this invention that compositions comprising a rare earth oxide and boric oxide, preferably together with minor amounts of phosphorus pentoxide, aluminum oxide and magnesium oxide are useful as sealing compositions in the construction of seals in ceramic discharge lamps, more especially between sintered ceramic oxides such as alumina, the cermet materials such as those disclosed in our co-pending Application DE-OS No. 26 55 726, for example alumina-tungsten and alumina-molybdenum cermets. Seals made with these compositions are, moreover, found to retain their hermetic properties and integrity without any sign of chemical attack after 100 hours at 900.degree. C. in reactive metal halide vapours such as mercuric chloride, mercury iodide, sodium chloride, tin chloride, scandium iodide, sodium iodide or cesium iodide.
Although the preferred rare earth oxide is lanthanum oxide, other rare earth oxides, such as Sm, Nd, Sc, Y, Yb, Dy or Ce oxides, or mixed rare earth oxides, can also be used.
Suitable sealing compositions comprise, by weight, 55 to 95% rare earth oxide, 5 to 45% boric acid, up to 5% phosphorous pentoxide, and up to 5% aluminum oxide and/or magnesium oxide.
The most successful compositions lie in the range of proportions by weight: 60 to 95% rare earth oxide, 5 to 50% boric oxide, 0 to 5% phosphorus pentoxide, 0 to 5% aluminum oxide and 0 to 5% magnesium oxide.
Also in accordance with this invention it has further been found that hermetic seals between dense alumina components, cermet components or alumina and cermet components can be prepared by using the composition defined above in vacuum or inert atmospheres between 1100.degree. C. and 1650.degree. C.
In particular, compositions around the eutectic compositions of the rare earth and boric oxides, for example 89.68% by weight lanthanum oxide and 10.32% by weight of boric oxide, have been found to seal translucent alumina are tubes to alumina-tungsten cermets at 1350.degree. C. The sealing material at the join between the sintered alumina and cermet components consists mainly of two major crystalline phases: in the case of lanthanum oxide 3La.sub.2 O.s
REFERENCES:
patent: 3588573 (1971-06-01), Chen
patent: 3615762 (1971-10-01), Parry
patent: 3682766 (1972-08-01), Maher
patent: 4122042 (1978-10-01), Meden-Piesslinger
Hing Peter
Khan Ehsan U.
Bell Mark
O'Connell Robert F.
Thorn EMI Limited
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